Immediate settlement

9.2.1 Cohesive soils

If a saturated clay is loaded rapidly, the soil will be deformed during the load application and excess hydrostatic pore pressures are set up. This deformation occurs with virtually no volume change, and due to the low permeability of the clay, little water is squeezed out of the voids. Vertical deformation due to the change in shape is the immediate settlement.

This change in shape is illustrated in Fig. 9.1a, where an element of soil is subjected to a vertical major principal stress increase Aa\, which induces an excess pore water pressure, Au. The lateral expansion causes an increase in the minor principal stress, Aa^.

The formula for immediate settlement of a flexible foundation was provided by Terzaghi (1943) and is

Excess pore pressure = A

Excess pore pressure = A

(a) Immediate settlement Fig. 9.1 Compressive deformation.

Excess pore pressure = O

(b) Consolidation settlement

Excess pore pressure = O

(b) Consolidation settlement

Table 9.1

L/B

Np

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